The present invention relates to a method and apparatus for controlling supplemental heat in a heat pump, wherein a controllable heating element is used in combination with the indoor fan coil to heat the supply air, and is controlled primarily based on the temperature of the air leaving the indoor fan coil, substantially independent of the temperature sensed at the indoor thermostat. The temperature of the air leaving the indoor fan coil can be sensed directly or predicted based on the sensed outside ambient air temperature.
Heat pump systems use a refrigerant to carry thermal energy between a relatively hotter side of a circulation loop, where compression of the refrigerant by a compressor raises the temperature of the refrigerant, to a relatively cooler side of the loop at which the refrigerant is allowed to expand, causing a temperature drop. Thermal energy is added to the refrigerant on one side of the loop and extracted from the refrigerant on the other side, due to the temperature differences between the refrigerant and the indoor and outdoor air, respectively, to make use of the outdoor air as a thermal energy source.
Heat pumps used in residential heating and cooling are bidirectional, in that suitable valve and control arrangements selectively direct the refrigerant through indoor and outdoor heat exchangers so that the indoor heat exchanger is on the hot side of the refrigerant circulation loop for heating and on the cool side for cooling. A circulation fan passes indoor air over the indoor heat exchanger and through ducts leading to the indoor space. Return ducts extract air from the indoor space and bring the air back to the indoor heat exchanger. A fan likewise passes ambient air over the outdoor heat exchanger, and releases heat into the open air, or extracts available heat therefrom.
These types of heat pump systems can operate only if there is an adequate temperature difference between the refrigerant and the air at the respective heat exchanger to maintain a transfer of thermal energy. For heating, the heat pump system is efficient provided the temperature difference between the air and the refrigerant is such that the available thermal energy is greater than the electrical energy needed to operate the compressor and the respective fans. The temperature difference generally is sufficient for efficient cooling, even on hot days. However, for heating when the outdoor air temperature is below about 25.degree. F., the heat pump system may be unable to extract sufficient heat from the outdoor air to offset the loss of heat from the space due to convection, conduction and radiation of heat from the structure to the outdoors.
When the heat pump is unable to provide enough heat to the structure (i.e., the outdoor temperature is below the balance point between the building load and the heat pump capacity) a supplemental heating element is provided in the supply air duct downstream from the indoor heat exchanger/coil to supply the additional heat required to maintain the desired indoor air temperature. Activation of the supplemental heating is typically controlled by an indoor thermostat, by which the occupants set a desired temperature to be maintained in the space by operation of the heating system.
Conventional heat pump control systems use a two-stage-heat/one-stage-cool room thermostat. On a first call for heat from the thermostat, the heat pump compressor and fans are activated to extract heat outdoors and to release the heat indoors. The heat pump supplies air to the structure (typically at about 80.degree. F.) until the indoor temperature reaches the thermostat set point (i.e., the first set point) and then is deactivated. If the heat loss of the structure is greater than the capacity of the heat pump, which occurs when the outdoor temperature drops, the indoor air temperature cannot be raised by the heat pump to the desired temperature. The indoor temperature thus continues to drop.
The room thermostat has a second switching means that is operated at a temperature slightly lower than the desired temperature at which the first switching means is operated. Conventionally, when the room temperature falls to the second set point defined by the thermostat, power is supplied to the supplemental heating element. The supplemental heating element supplies the additional heat needed to bring the indoor temperature up to the second set point temperature (typically the supply air is about 125.degree. F.), and thereafter the heat pump works alone to supply heat to the structure until the first set point temperature is reached.
As explained in U.S. Pat. No. 5,367,601, however, conventional two stage heat controls cause wide swings in the temperature of the supply air emitted into the structure by the heat pump system. Such wide temperature swings (e.g., 80.degree. F. to 125.degree. F.) are uncomfortable for the occupants and adversely affect the efficiency of the heat pump system. In an attempt to improve occupant comfort, the '601 patent proposes a control system that provides a closer control on the operation of the supplemental heating, by sensing the supply air temperature and then continuously controlling the on/off condition of the supplemental heating. While this proposal makes strides toward maintaining the supply air temperature at a given level, it has at least two significant drawbacks.
First, the supplemental heating is used only when there has been a second call for heat from the indoor thermostat. This makes it more difficult to maintain the air supply temperature at a constant, predetermined level, as supplemental heating is never energized during first stage heating.
Secondly, the temperature sensor must be positioned in the air supply duct of the building duct work by the technician installing the heat pump. Variations in the position of the sensor can lead to variations in temperature sensing accuracy, which in turn can lead to erroneous control of the supplemental heating by the controller.
U.S. Pat. No. 4,141,408 also discloses control means for controlling supplemental heating elements in a heat pump system. This patent proposes to use sensors positioned on the indoor coil to measure the temperature of the air leaving the coil. The sensors are connected to relays that close to operate one or two fixed output heating elements. This system is unable to prevent wide swings in the air supply temperature, because there is no means for operating the supplemental heating elements during first stage heating. There is also no means for precisely controlling operation of the heating elements, in that they are simply turned on and off in response to the temperatures sensed by the sensors.
U.S. Pat. No. 5,332,028 also discloses a heat pump system having supplemental heat for application to the supply air during periods of defrost operation in order to avoid a "cold blow" condition while the heat pump is operating in the defrost mode. This patent proposes to turn on a supplemental heating element in response to the sensed temperature of the supply air during defrost and responsively turn on additional heat in stages when necessary to maintain the temperature level of the supply air at a comfortable level during defrost. This system, however, also requires the installation technician to position the air supply temperature sensor and thus suffers from the same drawback as discussed above. And there is no means by which the supplemental heating elements are controlled precisely in order to avoid the wide air supply temperature swings mentioned above. Moreover, the supplemental heating elements are not operated during first stage heating in order to insure a constant air supply temperature at all times.